1. Field of the Invention
The field of the invention is electrical generators powered by internal combustion engines, and in particular, voltage regulators associated with such electrical generators.
2. Background Art
An electrical generator may be connected with an internal combustion engine to provide a means of generating emergency electrical power or generating electrical power at locations removed from electrical utilities.
When an A.C. generator is used, the speed of the engine is mechanically regulated to control the frequency of the A.C. electrical power, typically to 50-60 Hz. The output voltage of the generator is regulated by controlling the current in the field coil of the generator. Decreasing the field current, for a given load and speed of generator rotation, decreases the output voltage at the armature. Conversely, increasing the field current, for a given speed of generator rotation, increases the voltage at the armature.
The control of the field current for the purpose of controlling the generator output voltage is performed by a voltage regulator. Simple voltage regulators may be constructed from contacts that switch a resistance in series with the field windings to reduce the field current when the output voltage rises above a predetermined limit. Improved voltage regulators may be constructed from solid-state devices such as transistors or SCR's to provide continuous control of the field current in response to variations in the generator output voltage.
Considerable internal heat may be generated in the solid state device during regulation of the field current, depending on the capacity of the generator and its operating load. Higher wattage generators require voltage regulators having solid state devices with correspondingly higher power ratings to accept this higher heat load.
Voltage regulators used in electrical generators driven by internal combustion engines also experience an increased ambient temperature as a result of the externally generated heat from the internal combustion engine. Higher ambient temperatures require that the power dissipation ratings of the solid state devices be "lowered" to reflect the device's decreased ability to dissipate heat into the environment. Care may be taken to mount the voltage regulator in a manner to reduce the heat absorbed by the regulator from the engine. "Heat sinks" comprised of heat radiating air fins may be attached to the regulator to improve its ability to dissipate internally generated heat. Nevertheless, there may be little air flow to cool such heat sinks, particularly in marine-type engines where there is no radiator and hence no radiator fan to move air past the heat sink fins. Accordingly, an additional margin is ordinarily added to the specified power ratings of a solid state device used in voltage regulators operating in environments with elevated temperatures.
Solid state devices with high power ratings are more expensive. Further, within the design limits of a given solid state device, operation at elevated temperatures, either because of internally generated heat or externally generated heat, may decrease the operating life and reliability of the device.